Monumental buildings are the prestige part of the industry. This segment includes exterior grade sheets and components such as facades, curtain walls, panels and louvers. The coating to be used for these products is usually specified by the architect.

Light commercial buildings use the same exterior grade components as monumental buildings. However, they tend to be standard items produced on a larger scale by fabricators. The components themselves, not the coatings, are normally specified when the building is planned. The specific coating used can vary dependent on the supplier of the components and what kind of coating options and expertise they have.

The segment of the market referred to as "commodity" includes aluminum profiles and sheeting for products like storm doors and window frames. In Europe this is a large volume market, particularly on residential buildings.

Some exterior product is specified and used on the interior of buildings. These applications are typically areas where the coating could yellow from exposure to sunlight through glass windows.

Fixtures and fittings also must be coated. This is the catchall segment, which includes fencing, balustrades and detail work. The substrate can be aluminum, steel or galvanized steel. The coatings used in the United States are the same as for monumental and light commercial buildings.

Selection of a coating is usually focused on the appearance requirements, quality performance, cost and compliance. There are several coating options that are available for the aluminum architectural market. Common coatings used for architectural aluminum include anodizing, PVDF (Poly Vinylidene Fluoride), liquid paint and powder coating.

Anodizing. This was one of the first coating processes used on aluminum and has been used since the 1920s. The principle of the process is the conversion of the aluminum surface to aluminum oxide, which is long lasting, extremely hard, durable and corrosion resistant. The performance of the anodizing process is well known amongst architects and other specifiers and is the coating used on many of the best-known aluminum structures in the world, including the Sears Tower in Chicago.

Recently developed techniques in anodizing have made it possible to produce a broader range of colors. However, the color and gloss range is restricted when compared to spray coatings, with limits on special effects like textures. Cost per square foot and maintenance is reasonable compared to alternatives. The anodizing process uses a series of dip tanks so the material utilization is excellent. However, the chemicals used in the process include several acids and other hazardous products that present challenges in handling and disposal.

It can be difficult to achieve a consistent color across large surfaces in an anodizing process and it requires a level of process control that many companies are not able to achieve. An anodized surface can be stained by cement - often a problem in "fast-track" building where concrete may be poured at levels near to where the curtain wall is being installed below.

PVDF Coatings. These are solvent-based paints with a low solids level and come in two grades. The original form (brand name Kynar) contains 70 percent PVDF, the remainder being an acrylic resin. This grade has very high durability but comes at a high cost. The second, more cost effective, grades are based on a PVDF level of 50 percent. This grade has durability that is less than the 70 percent types but is still comparable to super durable powder coatings, meeting the AAMA 2604-98 weathering requirement of five years Florida. In spite of their excellent weathering characteristics, PVDF coatings tend to scratch and mar if abraded or scuffed. Fabricators and installers report significant levels of "on-site" damage that requires touching up and repair.

PVDF coatings are expensive both in material and application costs. The key difference in terms of application costs is that PVDF is applied as a multi-coat system. The minimum is a two-coat finish, but such PVDF coatings can only be produced as a matt finish. If higher gloss levels are required, one or more additional layers of clear topcoat must be applied. This increases the cost substantially.

PVDF has exceptional resistance to fading from sunlight. In addition, they can produce high "metallic" appearances, a very popular look on the curtain wall of high-tech high-rise towers.

Liquid Paints. These include solvent-based acrylics, polyesters, polyurethanes, and this technology in the architectural field is used in a significant volume in North America but in much lower volume in the rest of the world. Typical liquid paints used are very low cost and applied at low film weights (1 to 1.5 mil). These coatings are typically a low quality finish with poor appearance and performance. They are suitable for a mass market where cost is of the highest importance. A higher quality liquid coating with a suitable primer adds significant cost and environmental problems to the application process.

Although high solids technology has been successfully introduced, this does not eliminate VOCs completely. Water-based has not been widely adopted in this market, and therefore compliance remains a problem.

Powder Coatings. These have long been recognized for excellent finish quality in appearance and performance, reasonable cost per applied square foot, and relative ease of application in many industries. Application of organic powder coatings on aluminum extrusion for the architectural market has been an area of increasing interest for the industry for several years. Some early attempts to put powder into harsh outdoor environments were not very successful. But development in powder technology has advanced considerably in the last decade. In recent times, through resin development, it has become possible to produce powder coatings with exterior durability that can meet the AAMA 2604 specification (i.e., five years Florida exposure), making it an excellent choice for architects in North America. As a result of this progress, powder coatings are now regularly specified and used in extremely hostile environments like Australia, Hong Kong, South China and Malaysia.

Developments in manufacturing techniques have also meant that the common silver metallic effects favored in architecture have been achieved in powder coating. Architects in Europe and Asia have already specified these finishes in projects. Powder technology has also developed finishes, like textured finishes, that are unique to powder. In other parts of the world these finishes, have been adopted in architecture, particularly for internal curtain walling and partitioning, railings, doors and entranceways.

Historically, the aspect of powder coating that has had the most impact on wide spread use is the absence of solvent content, providing a unique environmental advantage. High solids liquid coatings and waterborne technologies have improved tremendously in reduction of emissions and waste products over recent years but still come up short in comparison to a product that is very near 100 percent solids and can be applied in a single coat with good performance properties.

There are additional benefits of powder coatings that can be considered relevant. Powder coatings are extremely tough, particularly compared to PVDF and the liquid paints used in this area. Already this benefit has been appreciated in some projects in North America. In these cases powder coatings have been selected in either heavy traffic areas (store fronts, hotels etc.), or complete buildings due to the enhanced toughness compared to PVDF.

Another strong advantage of powder coatings, particularly in the context of the architectural area, is the wide range of available colors, gloss variation and special finishes, like textures that are available.

The chemical resistance of powder coatings is also superior to most other finishes. For example, powder has superior resistance to alkalinity and therefore is not stained by concrete. This makes it suitable for usage in "fast-track" building where concrete may be poured at levels not very distant from where a curtain wall is being installed below.

In North America, most of the commodity type products are currently liquid coated with waterborne or solvent-based coatings. Light commercial components, typically standard items produced on a large scale by fabricators, are finished with liquid coating, PVDF or anodizing. Some of these manufacturers have gone to powder coating but many still use coatings with some solvent content.

The monumental building products are most often coated with anodizing or PVDF to meet AAMA 2605 (American Aluminum Manufacturers Association) requirements for weathering. But there is a significant difference between the rest of the world and the North American market (Table 1). In Europe, 50 percent of the total architectural aluminum market and as much as 18 percent of the aluminum sheet on the exterior of buildings is powder coated, while in the US the total percentage is around 6 percent. Clearly, there is an opportunity for more widespread use of powder coatings in this market.

Asia continues to use a large percentage of anodized aluminum. In North America the market tends to use much more liquid paint, PVDF and anodized finishing than powder coating while in Europe powder has emerged as the dominant finish. The fact that Europe is using so much more powder coating on building aluminum than the United States is related to several factors. First of all, there are many architects in the Unites States who are not familiar with powder coating and they may tend to specify the finishes that they understand such as anodizing and PVDF. Lack of awareness of the properties of powder coatings generally, and their design possibilities in particular, also factor in their lack of growth in the monumental field.

Also, recent advances in the quality of powder coatings are not well known or understood. This is understandable because powder coatings did not have the durability performance (weathering) to meet the old AAMA 605.2-92 specification until the last few years, while PVDF has been around for 25 years and did meet the AAMA specification.

In addition, some powder coatings have been applied to architectural aluminum products without proper concern for the pretreatment process, the specific performance qualities of the powder used and the applied film thickness. Because different coaters may use different methods of preparation and different powder coating materials applied at different film thickness the exterior durability of the coating may not always be acceptable. A powder coated part may show excessive fading of color and gloss due to the selection of the coating and poor pretreatment, leading architects to shy away from the use of powder on their products. It is possible to produce a powder coated part that will meet AAMA 2604 and even AAMA 2605 but the powder coating material and process must meet certain standards. Powders can meet the specifications but it is critical that the powder material and process are up to the level of quality needed for a particular installation.

When an architect does specify powder coating it is often for interior products such as railing, not exterior architectural elements. Also, there are lingering perceptions that powder is more expensive or difficult to apply than liquid coatings, even though the evidence is overwhelming that this is exaggerated and powder can be very competitive. Europeans may also have different standards and expectations for acceptable weathering over time and the importance of using an environmentally friendly coating.

What Comes First, Supply or Demand?

Another factor that seems to limit powder use here in the Unites States is the lack of coaters dedicated to running long aluminum extrusions in powder, particularly in high production vertical systems. Horizontal systems cannot produce the volume on long extrusions needed nearly as fast or cost effectively as vertical systems. In Europe the large vertical systems are quite common but there are very few facilities in North America.

It stands to reason that if architects do not specify powder coating for architectural aluminum there will not be many companies in the business of applying it. So what comes first, the architects ask for it or the coaters supply it even though the demand is low? One cannot happen without the other but it seems that the specification stage must come first to stimulate the demand.

Specifications for Finishing on Architectural Aluminum

Specifications also play a role in the decision process. Many Europeans follow the Qualicoat class 2 specifications or GSB while the US uses the AAMA specifications.

AAMA is the American Architectural Manufacturers Association. They supply three voluntary finish specifications:

• AAMA 2603-98: Pigmented Organic Coatings (one year Florida)
• AAMA 2604-98: High Performance Organic Coatings (five years Florida)
• AAMA 2605-98: Superior Performing Coatings (10 years Florida)

There is no Approving or Quality Labeling scheme, but products may be "listed" as certified to meet one or more of these specs.

GSB is a European Specifying, Approving and Quality Labeling body for coating of aluminum building components, serving German, Austrian and Swiss markets, and a handful of customers in other selected markets (e.g., Spain, Benelux).

Qualicoat is a European Specifying, Approving and Quality Labeling body for coating of aluminum building components, serving most European markets, including Germany. There are two specs: Qualicoat 1 (one year Florida) and Qualicoat Class 2 (three years Florida). Qualicoat schemes are also in use in Australia, and certain other countries in Asia and the Middle East.

The AAMA specifications are written differently and point to performance rather than process. It is a voluntary specification that does not require a license. The architect specifies that the coating must meet the AAMA specification and the supplier warrants that it will.

The GSB and Qualicoat specifications provide details for test methods, work specifications, approval of coatings, licensing of coating facilities, and in-house control procedures. A Qualicoat approval allows a facility to put a quality label on their architectural aluminum products. In order to receive a license a facility must be inspected twice and then they must be inspected at least twice a year to maintain their approval (Table 2).

When to Use Powder Coating

So what is the right coating for a given application and how does powder fit in? Can powder coating be used for a higher percentage of extruded architectural aluminum in the Unites States? Does powder perform as well as the alternative coatings? How does powder compare to other options in cost and other criteria? What about the process of application, is powder more or less difficult than liquid alternatives?

There is no doubt that powder coating could be used for more of these products. In many cases it would provide a better product at a lower cost with less impact on the environment. Powder has many positive properties compared to other coatings. For example, it often has better mechanical properties than PVDF and it has better mortar resistance than anodizing. With the significant strides in improved weathering in super durable powder coatings there are compelling reasons for increased use in the Unites States.

Selecting Powder for Outdoor Durability

If a powder coating will be used for exterior applications then the powder material and coating process must be qualified for production of exterior durable finishes. Powder material suppliers can produce data from Florida Long-Term testing to qualify a powder coating material. If a coater has experience with outdoor products they should be able to produce documents and references to demonstrate their ability to produce to the required performance standard. If this kind of documentation cannot be provided then another coater or coating material should be selected.

In reality, there are many instances where powder is the best option for a building project and other instances when it is not. But in order for architects to specify the correct coating they need to know more about the properties of each option and the impact of selecting one vs. another. Kynar will provide the superior weathering in those conditions that absolutely need to meet the AAMA 2605-98 spec but there are many advantages to using a powder coating on any other surface, including product that must meet AAMA 2604-98. Liquid paint is somewhat lower in cost but generally inferior to powder coating in performance. Anodizing is durable but limited in color and texture options and powder has better chemical resistance. With the overall advantages of VOC compliance, reasonable cost and excellent performance, powder coating offers significant opportunities to the architect.